Electrical steel sheet with axes of easy magnetization of crystals aligned in substantially the same direction in the steel sheet as a whole and high in crystal orientation is called “grain-oriented electrical steel sheet”. The direction matches the rolling direction of the steel sheet. Such a steel sheet is extremely superior as the material of a transformer core.
The Watt loss when magnetizing grain-oriented electrical steel sheet by an alternating current is divided into eddy current loss and hysteresis loss. Further, the eddy current loss is divided into classical eddy current loss and abnormal eddy current loss. Classical eddy current loss is proportional to the thickness of the steel sheet, so is reduced by making the material thinner. Abnormal eddy current loss is loss due to an eddy current locally generated due to movement of the domain walls and becomes smaller proportionally to the inter-domain wall distance of the magnetic domains narrow in the rolling direction, that is, the 180° magnetic domains. Therefore, to reduce the Watt loss, various technologies for refining the magnetic domains have been invented.
It is known that by imparting linear, cyclic strain substantially vertical to the rolling direction to the surface of steel sheet, thin circulating magnetic domains are formed in their vicinities, the 180° inter-domain wall distances become narrower starting from those points, and the abnormal eddy current loss is reduced. Therefore, the method of focusing the laser beam and scanning the sheet in the width direction to impart strain has been invented and is currently being practically used.
On the other hand, hysteresis loss is loss due to the magnetization curve, that is, the hysteresis curve, and is a Watt loss component sensitive to strain of the steel sheet. Therefore, there was the problem that imparting excessive strain by firing a laser leads to an increase in the hysteresis loss.
Further, along with Watt loss, an important property of the electrical steel sheet is magnetostriction. This is due to the expansion and contraction of the steel sheet in an alternating current field and is the main cause of noise in transformer products. In particular, it is known that in electrical steel sheet with a high crystal orientation, the amount of expansion and contraction of the steel sheet has a positive correlation with the amount of strain introduced. From the viewpoint of the magnetostriction, excessive strain has to be suppressed. Accordingly, it is desirable to use as small a strain as possible to reduce the abnormal eddy current loss and greatly suppress the increase in the hysteresis loss and magnetostriction.
In conventional Watt loss reducing technology firing a laser to impart residual stress, for example, as disclosed in Japanese Patent Publication (A) No. 6-57333, the method of firing a high peak pulse laser by a short pulse of 1 to 2 μs or so in order that the peak power density on the surface of the electrical steel sheet exceeds 1×104 W/mm2 effectively introduces strain. A high peak power Q-switch laser is being used. However, with this method, a locally extremely strong impact is given to the steel sheet, so relatively strong strain is imparted over a broader range than the focused diameter of the beam. As a result, the eddy current loss is sufficiently reduced, but the problem arises that the excessive strain causes the hysteresis loss and magnetostriction to increase.
Therefore, to introduce effective strain into a narrower region, for example, as disclosed in WO2004/083465, the rolling direction diameter of the focused spot of the laser beam is made 0.2 mm or less so as to impart strain to an extremely narrow region and obtain superior properties. With this method, compared with the high peak pulse laser, there is less occurrence of excessive strain width, but further improvement of the magnetostriction property has been desired. However, when further reducing the focused width, the power density on the electrical steel sheet surface increases, so even with a relatively low instantaneous power continuous wave laser, excessive strain enters. Further, in the case of a continuous wave laser, there is the problem that the continuous heat input process causes the steel sheet to easily melt. In this case, there was the problem that at the time of resolidification of the melted part, excessive tension occurred and rather the strain region increased. That is, in the continuous wave laser method, there were limits to the improvement of the magnetostriction property by just reducing the focused diameter.
In recent years, from the viewpoint of energy saving and environmental problems, the need for materials for high efficiency transformers, that is, high class electrical steel sheet, has been increasingly growing. In particular, due to restrictions on installation locations, the demand for reduction of transformer noise is high. Therefore, to reduce the Watt loss, technology for further improving the magnetostriction is desired.